This invention relates to a dry planographic printing plate and a method of making same. More particularly, it is concerned with a new type of a negative working presensitized planographic printing plate for use in dry planography and a method of making same.
Heretofore, there have been made several proposals as to a negative working dry planographic printing plate comprising a silicone rubber layer which overlies a photosensitive layer overlying a base substrate.
For example, in Japanese Patent Publication No. 16044/1971 there is proposed a presensitized planographic printing plate comprising a silicone rubber layer adhered by an adhesive layer to a light solubilizable diazonium type photosensitive layer overlying an aluminum plate. The plate is exposed to actinic light through a negative film to solubilize the photosensitive layer of the exposed area, then using a developer the solubilized photosensitive layer is dissolved off and at the same time the overlying silicone rubber layer is removed to allow the aluminum plate as a base substrate to be exposed as an image area.
Also in Japanese Patent Laid Open Publications Nos. 8306/1974 and 7320/1974 and Japanese Patent Application No. 131797/1978 there is proposed a negative working dry planographic printing plate having the same construction as above wherein the solubilized photosensitive layer and the overlying silicone rubber layer are removed by development and the exposed portion of the base substrate constitutes an image area.
In those proposed printing plates, the image area has a similar structure to that of a deep etched plate wherein the ink receiving portion consists of recesses (hereinafter referred to as "cell(s)") formed by the removal of the solubilized photosensitive layer and the overlying silicone rubber layer. In such a printing plate, it has heretofore been necessary to feed a larger amount of ink onto the printing plate in order to let ink adhere sufficiently to the cell portion at the time of printing. This, however, results in a large increase in the volume of ink consumption, leading not only to an increase in the printing cost but also to a serious problem in point of printing effect, namely in the reproduction of gradation. For example, in the case of feeding ink onto a printing plate so as to give a desirable optical density of the so-called solid portion of the resulting printed matter, the ink is not sufficiently transferred from the form roller of a printing machine into the cells on the plate forming a dot image of the so-called highlight portion of the printed matter, particularly into the cells forming very small dots of a diameter of the order of several ten microns. Thus the dots of the printed matter tend to get out of shape, or sometimes dot omission occurs in the printed matter due to no transfer of ink into the cells.
On the other hand, if the amount of ink to be fed onto the plate surface is increased in order to eliminate such disorder of dot shape, the optical density of the solid portion becomes too high with loss in printing effect balance, or sometimes there may occur a serious problem associated with the print quality, for example, the dots in the shadow area are not reproduced.
To solve these problems, it may be effective to reduce the film thickness of the photosensitive layer to be dissolved off and thereby make the cell depth smaller. However, it is very difficult and not practical to uniformly form a photosensitive layer having a thickness required to solve the foregoing problems associated with printing, e.g. a film thickness of about 0.1.mu., and further, thinning the photosensitive layer results in an enhanced physical and chemical influence from the base substrate, for example, the image reproducibility and the preservability are badly influenced in many cases.
Thus, the negative working dry planographic printing plates so far proposed have drawbacks in point of ink adhesion, etc. and are less practical.